Divergent Synthesis of Oxa-Cyclic Nitrones through Gold(I)-Catalyzed 1,3-Azaprotio Transfer of Propargylic α-Ketocarboxylate Oximes: Experimental and DFT Studies

Generation of Carbonyl Compounds from Oximes through Electrooxidative Deoximation

An electrochemical protocol was developed to generate carbonyl compounds from oximes with excellent functional group compatibility. Mechanistic experimental studies support the idea that an electrooxidative pathway of oximes might be involved, in which the water may serve as an oxygen nucleophile and the oxygen source for the final carbonyl compounds.

Access to Chiral O,O-Acetals Enabled by Palladium-Catalyzed Asymmetric Addition of Oximes to Alkoxyallenes

Enantiomerically pure acyclic O,O-acetal compounds (up to 97 % ee) have been accessed through chemo-, regio- and enantioselective palladium-catalyzed addition of oximes to alkoxyallenes. DFT calculations support that a protonative hydropalladation pathway is favourable, in which the hydrogen bonding interaction between the amide group of the diphosphine ligand and the alkoxyallene is critical for the highly stereoselective formation of the dioxygenated stereogenic center.

Gold(I)-Catalyzed Ring-Closing Alkyne-Carbonyl Metathesis for the Synthesis of Butenolides

Alkyne-carbonyl metathesis is a type of carbon-carbon forming reaction involving the construction a carbon-carbon double bond and a carbonyl group in one transformation. Herein, a Au(I)-catalyzed ring-closing alkyne-carbonyl metathesis protocol has been developed to make densely substituted γ-butenolides from propargyl α-ketoesters. It features 100 % atom economy, excellent substrate flexibility and benign functional group tolerance. Mechanistic studies demonstrate that the coordinative interaction between the gold catalyst and the alkyne might initiate the transfer of an oxygen atom and the formation of the carbon-carbon double bond. By using this gold-catalyzed ring-closing alkyne-carbonyl metathesis as a key step reaction, four naturally occurring butenolide-type compounds including decumbic acid (45 % yield for 3 steps), deoxyisosporothric acid (32 % yield for 5 steps), lichesterinic acid (34 % yield for 5 steps) and isomuronic acid (6 % yield for 8 steps) have been synthesized starting from commercially available starting materials.

Construction of Cyclic Nitrones Enabled by Photodriven and Gold-Catalyzed 1,3-Azaprotio Transfer of Allenyloximes

A protocol was developed to construct five- to seven-membered cyclic nitrones through the gold-catalyzed 1,3-azaprotio transfer of allenyloximes under photoirradiation. The photoisomerization of oximes was suggested to convert the inert stereoisomer to a reactive one. This photodriven and gold-catalyzed ring formation could be further extended to the thermodynamically stable aryl ketoximes with an E-configuration, which previously displayed chemical inertness in the absence of light irradiation.

Construction of 4-Acyl-2-quinolones through Sc(OTf)3-Catalyzed Ring-Closing Alkyne-Carbonyl Metathesis

An atom-economical protocol to construct densely substituted 4-acyl-2-quinolones from N-(2-alkynylphenyl)-α-ketoamides has been developed through Sc(OTf)₃-catalyzed ring-closing alkyne-carbonyl metathesis. Mechanistic experimental studies support that coordinative interaction between Sc(OTf))₃ and the substrate, the formation of an oxetene intermediate, and an electrocyclic ring-opening of the oxetene might be involved.

Mechanistic insights into nickel- and gold-catalyzed diastereoselective [4 + 2 + 1] cycloadditions between dienynes and diazo compounds: a DFT study

Density functional theory (DFT) calculations were performed to gain an in-depth mechanistic understanding of Ni(0)- and Au(i)-catalyzed diastereoselective [4 + 2 + 1] cycloadditions between dienynes and diazo compounds.

Chemodivergent Synthesis of Oxazoles and Oxime Ethers Initiated by Selective C-N/C-O Formation of Oximes and Diazo Esters

Chemodivergent reactions of oximes and diazo esters involving Rh-catalyzed [3+2] annulation and photodriven O-H insertion have been developed to generate oxazoles and oxime ethers. A range of aldehyde and ketone oximes reacted with α-diazocarbonyl compounds in a controllable manner in which functional groups, including ketone, ester, amide, ether, thiol ether, silane, alkene, allene, and alkyne groups, were well tolerated.

Copper-Catalyzed β-Lactam Formation Initiated by 1,3-Azaprotio Transfer of Oximes and Methyl Propiolate

A copper(II)-catalyzed protocol to construct trans-configured β-lactams and spirocyclic β-lactams from oximes and methyl propiolate has been developed, which features excellent substrate flexibility and diastereoselectivity (up to >99:1 dr). In situ FT-IR mechanistic experiments support that ketene species might be involved in the formation of β-lactams.

Iodoarene-Catalyzed Oxyamination of Unactivated Alkenes to Synthesize 5-Imino-2-Tetrahydrofuranyl Methanamine Derivatives

Reported here is the room-temperature metal-free iodoarene-catalyzed oxyamination of unactivated alkenes. In this process, the alkenes are difunctionalized by the oxygen atom of the amide group and the nitrogen in an exogenous HNTs₂ molecule. This mild and open-air reaction provided an efficient synthesis to N-bistosyl-substituted 5-imino-2-tetrahydrofuranyl methanamine derivatives, which are important motifs in drug development and biological studies. Mechanistic study based on experiments and density functional theory calculations showed that this transformation proceeds via activation of the substrate alkene by an in situ generated cationic iodonium(III) intermediate, which is subsequently attacked by an oxygen atom (instead of nitrogen) of amides to form a five-membered ring intermediate. Finally, this intermediate undergoes an S_N2 reaction by NTs₂ as the nucleophile to give the oxygen and nitrogen difunctionalized 5-imino-2-tetrahydrofuranyl methanamine product. An asymmetric variant of the present alkene oxyamination using chiral iodoarenes as catalysts also gave promising results for some of the substrates.

Oxaazabicyclooctene Oxides, Another Type of Bridgehead Nitrones: Diastereoselective Assembly from Acetylene Gas, Ketones, and Hydroxyl Amine

Unique bridgehead nitrones, 8-oxa-6-azabicyclo[3.2.1]oct-6-ene 6-oxides, have been assembled diastereoselectively via acetyldihydropyrans, products of one-pot self-organization of two molecules of ketones and two molecules of acetylene, which after oximation undergo acid-catalyzed ring closure. The proposed mechanism includes the enol double-bond protonation, followed by intramolecular cyclization involving the interaction of the carbocation formed with a nitrogen atom. A broad range of substrates tolerate this facile transformation, in which the bridgehead nitrones were isolated in high yields.